Selective call receivers such as pagers typically operate by receiving paging signals comprised of radio frequency signals modulated by digital data in a predetermined format, sent from a paging terminal and transmitter. A paging system is comprised of a plurality of such pagers, all operating on the same radio frequency and all with the same digital format, and a paging terminal and transmitter, generating and sending paging signals with the proper digital format and radio frequency.
Since most pagers within a system are battery-operated, battery-saving techniques are usually employed in order to maximize the life of the battery. These battery-saving techniques typically comprise activating the radio frequency receiver section within each pager only during times when it is known that a page might be received. Each pager in a system periodically strobes its radio frequency section "on" to check for the presence of paging signals at the proper radio frequency and modulated with the appropriate digital format. If such a signal is detected by a pager, the radio frequency section within the pager is held "on" for an extended period of time to enable the pager to receive any pages that might be destined for it.
In some paging systems, there is, on a specific single radio frequency, a mixture of pagers with various digital formats. This requires the paging terminal in these systems to alternate the digital format with which the sent radio frequency signals are modulated. In some pagers in the system, this requirement can cause a problem of the radio frequency section being held "on" for a signal with a digital format other than the proper one. This phenomena is known as "falsing." The falsing phenomena is known to be especially prevalent in paging systems that mix the Golay Sequential Coding (GSC) and Post Office Code Standardisation Advisory Group (POCSAG) codes.
The GSC code comprises a single digital format that operates at two data rates, 300 baud and 600 baud. The POCSAG code comprises two separate and distinct digital formats, one of which operates at 512 baud, the other of which operates at 1200 baud. The incompatibility between the GSC and POCSAG formats results from the fourth harmonic of 300 baud GSC and the second harmonic of 600 baud GSC being recognized as a legitimate signal on 1200 baud POCSAG pagers. As a consequence, 1200 baud POCSAG pagers can recognize extraneous 300 baud or 600 baud GSC signals as the proper digital format and, therefore, unnecessarily extend the "on" time of the radio frequency section of the receiver, thereby degrading battery life. Similarly, the 600 baud GSC signal or the second harmonic of the 300 baud GSC signal can be recognized as the proper format on a 512 baud POCSAG pager.
A known method in the current art for preventing POCSAG pagers from falsing on GSC signals comprises the Beesley method. In the Beesley method, a decision is made whether to extend the radio frequency section "on" time based upon a sample of the incoming digital data performed over a predetermined number of transitions, where a transition comprises a change in the incoming data from a zero to a one or vice versa. If the elapsed time between any pair of sampled adjacent transitions is greater than 1.5 bit-times (at the desired baud rate), the elapsed time between those two adjacent transitions is normalized down in increments of one bit-time until it is less than or equal to 1.5 bit-times. Sampled pairs of adjacent transitions that are less than or equal to 1.5 bit-times apart are not normalized. At the end of the data sample, the result is a predetermined number of samples that are between 0 and 1.5 bit-times. These samples are then averaged in order to calculate an estimated baud rate and, if the estimated baud rate is within preset limits relative to the desired baud rate, the radio frequency section is left "on" for an extended period of time. The preset limits relative to the desired baud rate are typically predetermined and are stored in the memory of the pager.
The principle shortcoming of the Beesley method is its inability to reject extraneous signals that are integral multiples of the desired baud rate. For example, if a 600 baud one/zero pattern is present in a 1200 baud POCSAG Beesley system, each adjacent pair of transitions will be two bit-times apart at 1200 baud (1200/600=2). The sample, then, between each pair of adjacent transitions will be normalized down one bit-time to be equal to one bit-time (2-1=1). Therefore, after the predetermined number of transitions, the averaged value of the samples will be 1 bit-time and, therefore, it will appear that a 1200 baud signal is being sent. The radio frequency section of the pager will then be left "on" for an extended period of time unnecessarily, thereby degrading the battery life of the pager.
An additional shortcoming of the Beesley method is the inflexibility of the preset limits relative to the desired baud rate. Depending on the exact mixture of data rates and formats being sent by the transmitter, this inflexibility can result in an undesirable level of falsing.
Thus, what is needed is an improved method of rejecting extraneous signals in a paging system, particularly in systems that use both the GSC and POCSAG formats.